Technology:Stabilization using the Sol Gel Process
- The Sol Gel processing is a general synthesis technique that uses hydrolysis and condensation to produce solid matrices from liquids
- Ceramic portion formed after tetraethlyorthosilicate (TEOS) was prehydrolized with acidified water (0.15M HCL) in tertrahydrofuran (THF)
- The polymer polybutadiene was added and the solution was refluxed for 30 minutes
- Salt waste surrogate was mixed into the solution and stirred until the solution thickened
- Solution was then transferred to a plastic container, allowed to gel, then capped (the cap was punctured with small holes to allow gas to escape) and dried in an oven at 66C for a minimum of 24 hours, then placed in a vacuum oven at 70C for three hours
- The resulting material was a polyceram waste form
- Process modified after initial test results showed open porosity in sample waste forms; to minimize open porosity, dried samples were submerged in a polycream or resin solution and placed under vacuum to allow infiltration, then dried overnight

Contaminants:Metals and salts
- Two salt-containing, nonradioactive surrogates - one with nitrate salts; one with chloride and sulfate salts
- Both contained 1,000 mg/kg each of lead, chromium, cadmium, and nickel (in the form of metal oxides)

Purpose/Significance of Application:Laboratory testing of the sol gel process to stabilize high salt content waste

Regulatory Requirements/Cleanup Goals:
- RCRA TCLP criteria for metals
- Leachability index (LI) of at least 6.0 for the salt components
- Compressive strength of salt waste forms of at least 60 psi
- Final waste form must incorporate at least 10-wt% of the salt component

Results:- Initial samples met requirements for compressive strength and LI; however, forms contained open porosity which exacerbated leaching, resulting in the samples not meeting the TCLP limits for metals
- After process was modified to minimize open porosity, samples were below the TCLP limits for all metals and very near or below the UTS limits for metals (results for cadmium and chromium were reported slightly above the UTS limits, but results were below the practical quantification limits of the instrument)
- The second waste form samples contained 50% of the chloride/sulfate salt surrogate; data on compressive strength and LI were not available; however, report indicated that these samples were expected to be stronger and have a higher LI than the first samples

Cost Factors:- To date, no detailed cost analyses have been performed on this process
- The report included an order of magnitude estimate for the Sol-Gel process in the range of $600,000 to $1 million for design, capital equipment, installation, and startup costs, as well as obtaining the required environmental and operating permits

Description:At the Pacific Northwest National Laboratory, DOE conducted laboratory scale testing of the Sol Gel process to stabilize high salt content waste. Two salt-containing, nonradioactive surrogates - one with high levels of nitrate salts and one with high levels of chloride and sulfate salts - were used for the tests to simulate wastes at DOE facilities. The Sol Gel process involved combining a polymer (polybutadiene) and an oxide-based ceramic (formed using TEOS, acidified water, and THF) to produce a solid material referred to as a polyceram. The resulting polyceram waste forms were tested to determine leachability and compressive strength at salt waste loadings of at least 10-wt%.

While initial samples met the requirements for compressive strength and leachability index, they did not meet the TCLP criteria because the form contained open porosity. To minimize open porosity, the process was then modified to include infiltration of dried samples with a resin. Test results for the infiltrated samples were below the TCLP levels and near or below the UTS levels. While a detailed cost analysis had not been performed on the process, an order of magnitude estimate indicates that the process would cost in the range of $600,000 to $1 million.